JP1999(11)-287258A discloses a driving force transmitting apparatus by which a driving force is transmitted by engaging a frictional clutch. This driving force transmitting apparatus is mounted on a four-wheel-drive (4WD) vehicle and distributes a driving force of an engine not only to front wheels but also to rear wheels as needed. In this driving force transmitting apparatus, a cam mechanism is operated in response to an actuation or engagement of a pilot clutch so that a main clutch is actuated or engaged by a thrust force generated at the cam mechanism. This main clutch includes outer clutch plates and inner clutch plates. The outer clutch plates are in spline engagement with or are spline-fitted with a first torque-transmitting member that is located outside and is inputted with, for example, a driving force of an engine. The inner clutch plates are in spline engagement with or are spline-fitted with a member configured to transmit a driving force to an auxiliary driving wheel.
Each spline-engaged area between each outer clutch plate and the first torque-transmitting member includes a predetermined amount of backlash (play), and each spline-engaged area between each inner clutch plate and the member for transmitting a driving force to the auxiliary driving wheel includes a predetermined amount of backlash (play). Therefore, an axial directional movement of each outer clutch plate becomes smoother and lubricant flowing between the clutch plates is drained appropriately. When torque is not transmitted from the first torque-transmitting member to the second torque-transmitting member, the main clutch is not at an engaged state. For example, in a case where the driving force transmitting apparatus is applied for connecting and disconnecting a transmission of a driving force to rear wheels of a vehicle, when torque is not outputted to the rear wheels, the main clutch is not engaged. Therefore, the outer clutch plates and the inner clutch plates are not constrained and are allowed to move. As a result, when torque is not transmitted from the first torque-transmitting member to the second torque-transmitting member, the axes of the outer clutch plates and the inner clutch plates of the main clutch correspond to the axes of the first torque-transmitting member and the second torque-transmitting member, wherein the main clutch rotates in an eccentric manner relative to the first and second torque-transmitting members. Therefore, when the driving force transmitting apparatus rotates in accordance with a driving of a vehicle, rotational imbalance may be created and transmitted as vibration to an occupant. The occupant may obtain an unpleasant and unstable feeling.
The present invention has been made in view of the above circumstances, and provides a driving force transmitting apparatus, which prevents a looseness of outer clutch plates and inner clutch plates of a clutch assembly and reduce a degree of rotational imbalance even when torque is not transmitted between a first torque-transmitting member and a second torque-transmitting member.